Job sharing
A. No. Job sharers are part-time workers, protected by the Part-time Workers (Less Favourable Treatment) Regulations 2000. The regulations entitle part-time workers to be treated no less favourably than a comparable full-time worker as regards contract terms — for example pay or holidays — and not to be subjected to any other detriment by the employer because of their part-time status.
Not only are contractual rights protected, but also other benefits. These benefits and entitlements are pro rata to your part-time hours.
The regulations provide a defence where an employer can show that less favourable treatment is justified on objective grounds.
In your case, if you perform the same duties and responsibilities as before, simply spread over a shorter working week and shared with your colleague, it is difficult to imagine any objective justification for cutting your terms and conditions.
There can sometimes also be sex inequality issues with this kind of change. Most part-time and job share workers are women. Treating a female job share worker less favourably because of her part-time status can result in sex discrimination.